Two comets will appear to whiz past each other.

Like rocket ships passing in the night, a pair of comets will appear to whiz past each other in the sky Tuesday. Starting at 5 p.m. ET, armchair astronomers can catch the show live online via observatories on a Canary Islands (map)volcano.

Though invisible to the naked eye, comet 168P/Hergenrother and comet C/2012 J1 (Catalina)—discovered just last May—have both been faintly visible through backyard telescopes for the past few weeks. As they approach the sun, more of their ice is vaporizing, adding to their hazy, reflective envelopes of gas and dust; producing characteristic comet tails; and boosting their brightness.

Video: Watch Live Feed of Comet Rendezvous

"Two comets with developed tails, visibly heading in opposite directions and changing position while we watch and 'missing' each other by a mere three-quarters of one degree—I simply can't recall ever seeing this happen!" enthused Bob Berman, an astronomer and columnist with Astronomy magazine.

That three-quarter-degree separation—roughly the width of the full moon's disk, as seen via the naked eye—is a fortunate illusion of perspective for sky-watchers. In reality the void between the two comets is about twice as long as Earth's distance from the sun.